N-type semiconductors and P
pp 18-28
the student must:
Distinguishes semiconductors from pipes
and insulators
He knows that in semiconductors electricity
due to two types of operators, the electrons
and holes
Aware of specific zones and
the conduction band and valence band
The influence of enrichment
the doped conductivity type semiconductor.
K E F A L A I O The C T I O C A T
2.1 Intrinsic Semiconductors
2.1.1 Conductors, Insulators, Semiconductors
In everyday life we are used to separate materials according to their electrical properties in two categories: conductors and insulators. In addition we accept that the conductors are perfect and that is also perfect insulators. These assumptions are good enough to meet all the requirements of simple electrical circuits. Between good conductors and good insulators there is another class of materials which are good conductors and are not good insulators. These materials are called conventional semiconductors (semiconductors).
Semiconductors differ from conductors and insulators in two different ways:
a) The first is a measure of the resistivity of the semiconductor. The conductors and insulators, the resistivity depends only on the material and is about 1x10-8Qm the conductors and insulators in 1x1011Qm. Unlike semiconductors, the resistivity for a specific material such as silicon (Si) or gallium arsenide (GaAs), can vary widely limits from 1x10-6 to 1x106 Qm respectively. Thus semiconductors are not good conductors, but also is not good insulators. This explains the name.
b) The second is the significant dependence of the resistivity of the semiconductor temperature due solely, as we shall see, in how they determine the concentration of free electrons in the material. So for common circuits operating temperatures, 0 ° C to 80 ° C, the specific resistance of conductors and insulators are changed little when the temperature varies. In the case of metals there is an increase of resistivity with temperature. Unlike the resistivity of the semiconductor varies significantly with temperature and specific decreases exponentially.
To understand the electrical properties of conductors, insulators and especially of the semiconductor should consider the structure of atoms and their bonds with their neighboring atoms.
The most common semiconductor is silicon (Si) and germanium (Ge).
These two elements are in the fourth column of the journal
table and have several properties that are common to those of coal. They have four electrons in its outer layer and each one of these electrons is associated with an electron from any nearby neighboring atoms, which form four covalent bonds. So each person acquires eight electrons in its outer layer that allows the formation of stable bonds and the formation of crystals in which the atoms are in specific positions.
Figure 2.1.1 shows the
creation of covalent bonds of silicon or germanium with the contribution of electrons of the outer layer while the shape
2.1.1, is depicted in two dimensions, the crystal structure. It should be noted that similar
how to create the two known crystal structures of carbon, graphite and diamond.
When the electrons of the outer layer can be removed from the people there will be no free electrons in the material and therefore not possible and the passage of electricity through it. So in this case the material behaves as an insulator.
(A)
(B)
Figure 2.1.1
(A) The formation of four covalent bonds with electron contribution and (b) a representation of silicon or germanium crystal
In silicon and germanium, the size of atoms, connecting the outer electron layer with the person is relaxed. As a result, at room temperature, some electrons removed by thermal way by people (see Figure 2.1.2) and move freely through the crystal of the semiconductor until they meet another person from whom a missing electron.
The movements performed these electrons are completely random so that the average electric current through the semiconductor is zero. If the semiconductor apply an electric field, with the help of an external power source, free electrons will move under the influence of the field thus pass through the semiconductor an electrical current.
The maternal person has lost, due to thermal excitation, an electron is no longer electrically neutral and acquires a positive charge. Because the person can not be moved, the deficit of negative charge can be offset by "borrowing" an electron from a neighboring atoms, which then acquires a positive charge. This can then repeat with another person nearby, etc.
Free electron
In a homogeneous material all the atoms and electrons are the same as mentioned above the electron deficit, ie the positive charge moves in a random manner, such as free electrons. This "free" moving positive charge called a hole (hole).
The move will cut a hole when "epanasyndethei1 with a free electron and cancel each. This" reunion is the Mo-
Figure 2.1.2
Representation of a valence electron that is removed and can move freely
Semiconductors
smefsi the free electron by a person who has an electron deficit. When an applied electric field, the hole will move in the direction opposite to that of the electron. So in a pure semiconductor, in which all people are equal, the electric current that flows through and measured by an ammeter has two components: an electron and a power cord hole. In addition to a pure semiconductor the number of free electrons is equal to the number of free holes. Such a semiconductor is called intrinsic semiconductor (intrinsic semiconductor).
In a semiconductor the concentration (number / cubic cm) of free electrons and holes are not rising due to thermal excitation. O reconnection mechanism, which is proportional to the concentrations, leads to an equilibrium where the rates of generation and reconnection equalized. This process determines the concentrations of electrons and holes in a pure semiconductor at each temperature and hence the conductivity and resistivity at each temperature. It should be noted that in a pure semiconductor resistance decreases exponentially with temperature.
2.1.2 Energy bands semiconductor
The quantitative interpretation of the electrical, optical and optoelectronic properties of semiconductors can be done only with the help of the theory of "energy zones" (energy bands). Generally the energy bands allow us to better understand the mechanism that determines whether a material is an insulator, semiconductor or metal. The energy bands are the allowed levels, which can be occupied by electrons in a crystalline or amorphous material even.
To understand the concept of energy bands, without the use of quantum physics, we should bear in mind that an isolated individual electrons are distributed in different layers, which each electron is in a very specific energy level. In a system of many people, such as a crystal or other semiconductor material, the interaction resulting from the short distance between individuals and the contribution of each individual with one or more electrons to the creation of chemical bonds has resulted in widening of the energy layers and convert them into energy bands.
It is known that a person is the number of layers is large. O number of layers that are fully or partially occupied by electrons depends on the number of protons in the nucleus. It is also known that the capture of electrons in layers starting from those found
closer to the core, ie those with lower energy. Respectively in a crystalline material the lower energy bands are fully occupied with electrons. In an area such an electron can not move because all the possible positions are occupied. An electron can move only when there are free places at one energy band, ie when only part of it is occupied by electrons. The energy bands which are higher energy than the valence electrons are empty.
As a single person among the stacks are so energy gaps between energy bands are the corresponding energy gaps (band gaps), or restricted zones (forbidden bands), in which there are no authorized positions for electrons
(See sch.2.1.3). Therefore the electrons occupy positions only on the energy bands and energy jumps move from one zone to another, which is to absorb energy to go to a higher emission energy or to switch to a lower, if there are no free seats. This energy can be thermal or photon correspond to, eg the photoelectric effect and fotoagogimotita when absorbed energy or the emission of light in diode LED.
Of all the energy bands in a material, two are of particular
interest. This corresponds to the layer of valence electrons
called the valence band (valence band) and the next, which corresponds to a higher energy, called conduction band (conduction band). The width of the energy gap, which in Zone Lgogimotitos
Valence band
2nd Conditional Zone
1st Conditional Zone
Figure 2.1.3
Energy band diagram of a hardware
22
Semiconductors
tratai in electron volts (eV) and the percentage occupancy of the conduction band are determined by the material. Based on the energy bands and energy gaps is possible to redefine the terms insulator, semiconductor and metal.
(B) (c)
Figure 2.1.4
Energy bands in a (a) insulator, (b) semiconducting and (c) metal
In the case of an insulator such as diamond, see sch.2.1.4a the valence band is fully occupied and the breadth of the energy gap between valence and conduction band is large (6 eV). So it is not possible to thermal excitation of electrons from the valence band to the conduction band and the material is non-conductive, ie an insulator.
In the case of a semiconductor such as silicon, sch.2.1.4v see the range of the energy gap between valence and conduction band is not large (1.2 eV). In this event, the thermal excitation of electrons from the valence band to the conduction band and the conductive material is a few, namely semiconductor.
If a metal sch.2.1.4g see the range of the energy gap between valence and conduction band is zero, ie no overlap between the zones. This results in having a large number of free electrons and free seats available in the conduction band, which makes the material highly conductive metal that is.
GENERAL ELECTRONICS
23
2.2 Semiconductors impurities
The intrinsic semiconductors have equal concentrations of electrons and holes, for this reason their applications are limited and determined by the resistance that varies very strongly when the temperature changes when illuminated. If an intrinsic semiconductor to add a very small amount of an element of the third to the fifth group of the periodic table, the semiconductor acquires impurities (impurities). The process of adding impurities is called enrichment (doping) and enriched semiconductor material (doped semiconductor). In such a semiconductor in which impurities determine the concentrations of electrons and holes, the semiconductor ceases to be endogenous. Because the concentrations of electrons and holes longer defined by one exogenous factor, ie the impurities and the semiconductor is called extrinsic semiconductor (extrinsic semiconsductor). The type of dopant used will determine whether the concentration of electrons will be larger than the hole or vice versa. In the first case of the semiconductor is called n-type and second type P, by the fact that the loads agoun the electricity is electrons, ie negative (Negative) or holes, ie positive (Positive), respectively.
In such a semiconductor impurities occupy positions of atoms in the material and form links
2.2.1 N-type Semiconductors
The N-type semiconductors are created when a semiconductor such as silicon or germanium added a very small amount of an element of the fifth group of the periodic table. The data is usually used as impurities are arsenic, phosphorus and antimony while the quantity required is of the order of a few parts per million, ie one million people each silicon or germanium atoms are some arsenic or phosphorus.
The dopant atoms are incorporated in the crystal structure of the semiconductor, occupy positions of atoms and form covalent bonds with neighboring atoms. Because the people of the fifth group of the periodic table have five electrons in the valence shell, when they realize a person in a position of the semiconductor will use for four
24
Semiconductors
Free
electron
Si
fa) Si (b)
Figure 2.2.1
Germanium crystal lattice where a person has been replaced with a person antimony. (A) Structure Links (b) visual form
the formation of covalent bonds remain unsold and an electron, which orbits the nucleus of the impurity. The electron can at room temperature, removed much more easily than an electron in intrinsic semiconductor (see sch.2.2.1).
Because pentavalent impurity element "gives" the semiconductor electrons, called the donor (donor). Then the man of the impurity is ionized and becomes positively charged. Because the removal of an electron from the donor is much easier than from a person of the semiconductor, the "borrowing" an electron from a nearby person will be difficult. Moreover, the probability is near one another donor could easily "borrow" an electron is negligible. This leads to the positive charge is stationary on the donor and the semiconductor to move the free electrons. Of course, the removal of electrons from the atoms of the semiconductor should not be excluded but should be noted that their number is very small compared to the number of electrons from donors. Thus, the addition of donors has led to many free electrons exist and very few holes in semiconductor.
25
(B)
Figure 2.2.2
Germanium crystal lattice where a person has been replaced with a person indium. (A) Structure Links (b) visual form
Therefore, an N-type semiconductor the electric current carried mainly by one type of cargo, electrons, called the majority of players and the majority (majority carriers) Unlike the holes in the N-type semiconductors are called minority institutions and minority (minority carriers). Finally, increasing the concentration of donors in a semiconductor leads to increased concentration of electrons and hence the conductivity.
2.2.2 P-type Semiconductors
The P-type semiconductors are created when a semiconductor such as silicon or germanium, add very small amount of an element of the third group of the periodic table. The data are commonly used as impurities are barium, gallium and indium, while the quantity is required, as in N-type semiconductors, the order of a few parts per million.
The impurity atoms occupy the positions of the atoms of the semiconductor. Because the senior group of the periodic table have three electrons in valence shell, when they realize a person a position of the semiconductor, we use all the valence electrons to form covalent bonds. This will remain an adjacent semiconductor atom, which would require an electron to form the complete structure of eight electrons in the outer layer. The electron will be required to "daneistei11 from a neighboring atom semiconductor. The electron will occupy, in this way, the vacancy will ionize to a negative charge the man of the third group of the periodic table. This process corresponds to the" release "of a hole, and because these people accept an electron are called recipients (acceptors) (see sch.2.2.2).
